Refractors

The refractor telescope uses a lens to gather and focus light. The first telescopes built were refractors. The small telescopes sold in department stores are refractors, as well as, those used for rifle scopes.

Advantages

1. Refractor telescopes are rugged. After the initial alignment, their optical system is more resistant to misalignment than the reflector telescopes.
2. The glass surface inside the tube is sealed from the atmosphere so it rarely needs cleaning.
3. Since the tube is closed off from the outside, air currents and effects due to changing temperatures are eliminated. This means that the images are steadier and sharper than those from a reflector telescope of the same size.

Though excellent refractors are still made, the disadvantages of the refractor telescope have blocked the construction of very large refractors for use in astronomical research.

Disadvantages

1. All refractors suffer from an effect called chromatic aberration (``color deviation or distortion'') that produces a rainbow of colors around the image. Because of the wave nature of light, the longer wavelength light (redder colors) is bent less than the shorter wavelength light (bluer colors) as it passes through the lens. This is used in prisms to produce pretty rainbows, but can it ruin an image!
   There a couple of ways to reduce chromatic aberration. One way uses multiple compensating lenses to counteract chromatic aberration. The other way uses a very long objective focal length (distance between the focus and the objective) to minimize the effect. This is why the early refracting telescopes were made very long.
2. How well the light passes through the lens varies with the wavelength of the light. Ultraviolet light does not pass through the lens at all.
3. How well the light passes through decreases as the thickness of the lens increases.
4. It is difficult to make a glass lens with no imperfections inside the lens and with a perfect curvature on both sides of the lens.
5. The objective lens can be supported only at the ends. The glass lens will sag under its own weight.

Refractors are best for:

• Observers who want the best possible image quality